There are a number of methods for sensing the presence of speech. Most of these methods are based upon the measurement of strength of the incoming wave. However, strong interference can confuse such systems and falsely indicate that signal is present.
A number of alternative methods for sensing the presence of signals are described in patent application, Ser. No. 693,716. One of the systems disclosed is based upon the determination that the input wave resembles an expected noise or interference waveshape, and if it does resemble the waveshape the information is used to operate a circuit which squelches or attenuates the output wave. FIG. 3 of patent application Ser. No. 693,716 shows one circuit for accomplishing the waveshape comparison operation.
Other arrangements have been disclosed for improving squelch performance such as reissued patent, Re. 27,202 where the communications channel is split into two or more segments and means are provided for comparing the energy in each segment. The gain for each segment is adjusted so that for normal types of noise the energy in each segment is equal. Generally, the adjustment is made for white noise, but the adjustment can also be made for a constant amplitude tone. However, if the tone varies in frequency, the system requires constant readjustment.